Solutions for solid minimization in the sludge streamline of municipal wastewater treatment plants: Current state and recent developments

Sewage sludge (SS) production has become a crucial problem worldwide for municipal wastewater treatment plant (MWWTP) operators due to the problems related to the handling and disposal of the huge SS volumes annually produced. Therefore, the implementation of innovative strategies enhancing the reduction of SS volumes to be finally disposed is gaining remarkable attention. This review aims to present the current state and latest trends of the solid minimization approaches that can be integrated in the sludge streamline of a MWWTP, where the anaerobic digestion (AD) is the most widely used process for solid reduction. Such approaches include SS pre-treatments before AD, strategies enhancing AD operation and post-treatments of SS digestate. During this work, almost 200 literature studies were reviewed. The main goal was to provide a comprehensive discussion targeting the mechanisms, advantages and disadvantages of each process, with particular attention on the performance of each method towards solid reduction. The ability of the most recent full-scale applications in maximizing SS destruction are presented and the potential/limits of the technologies still under development are introduced. In addition, a comparative analysis, based both on economic and environmental aspects, is proposed to provide an exhaustive overview on the full-scale applicability of the most studied technologies, not only in terms of SS reduction efficiencies, but also from an economic and environmental point of view.